Impact of Recovery Characteristics on Switching Loss of SiC MOSFETs

The influence of the recovery characteristics on the switching behavior of SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) with different switching speeds was investigated. A comparative analysis of the devices with different recovery characteristics revealed an increase in the turn-on loss (Eon) owing to the higher output capacitance charge (Qoss) and reverse recovery charge (Qrr) in the recovery arm. On the other hand, a higher Qoss in the recovery arm resulted in a lower turn-off loss (Eoff). In addition, an increase in Qoss and Qrr further influenced Eon and Eoff at a higher switching speed. Furthermore, a higher Qrr observed at a higher switching speed indicated a more significant impact of Qrr on Eon at a high switching speed than that of Qoss. The findings clarified in this study highlight the necessity of focusing the recovery characteristics to ensure a desirable switching loss of SiC MOSFETs.


Introduction
Owing to their excellent material properties, SiC MOSFETs are regarded as promising candidates for future power electronics compared with commonly used Si insulated gate bipolar transistors (IGBTs). One of the main advantages of SiC MOSFETs over Si IGBTs is the significantly lower switching loss owing to the absence of the minority carrier storage. The resulting higher switching frequency operation of SiC MOSFETs in comparison with that of Si IGBTs enables the realization of a higher power density and lower energy consumption in the power electronics systems.
However, the switching loss of SiC MOSFETs could have a substantial impact on the total loss in high switching frequency operation [1][2][3]. Therefore, reducing the switching loss of SiC MOSFETs by increasing the switching speed for higher switching frequency operation is of significance.
Because of a higher Qoss of SiC devices than that of Si devices, which is attributed to the two orders of magnitude higher doping concentrations of the drift layers, the displacement current of the freewheeling diodes in the recovery arm during the switching process varies the drain current (Id) of the transistors in the switching arm, and thereby influences Eon and Eoff [4]. Moreover, the influence of Qoss on Eon is in a similar degree as that of Qrr, which is derived from the parasitic body diodes of SiC MOSFETs in the recovery arm [4], although Qrr dominates and Qoss is negligible in Si devices [5].
In this study, the influence of Qoss and Qrr on the switching loss of SiC MOSFETs with different switching speeds was investigated in order to clarify the impact of the recovery characteristics under a higher switching speed.

Experimental
Switching measurements were performed with a double pulse test using the equivalent circuit shown in Fig. 1. To clarify the influence of the recovery characteristics, three type of configurations, which are also depicted in Fig. 1, were investigated. The switching voltage was 600 V, which determines the Qoss value in the switching measurements because it is derived from the charging or discharging current of the output capacitance in the recovery arm. The devices used in the recovery arm are (A) a MOSFET, (B) a MOSFET with an SBD, and (C) a MOSFET with two SBDs. The difference in Qoss in the recovery arm of each configuration is displayed in Fig. 2(a), which is calculated using TCAD simulation. The higher Qoss of (B) and (C) than that of (A) represents the larger number of SBDs in the recovery arm. Fig. 2(b) shows the measured diode characteristics of the recovery arm for each configuration at room temperature (solid lines) and for the configuration (A) at 150 o C (a dashed line). The source-drain voltage of (B) and (C) is smaller than the built-in potential of the parasitic body diode of (A) up to the measured current range, indicating that the parasitic body diodes of (B) and (C) are not activated. The higher diode current of (A) at 150 o C than that at room temperature indicates the existence of minority carrier injection at a higher temperature. To clarify the difference in the minority carrier injection at different temperatures, the recovery waveforms of (A) at room temperature and 150 o C were evaluated by changing the switching current (Isw), as shown in Fig. 3. The reverse recovery current at room temperature shown in Fig. 3(a) is independent of Isw, indicating a negligible influence of minority carrier storage. Therefore, the observed reverse recovery current is derived only from the displacement current due to Qoss. On the other hand, as shown in Fig.  3(b), a higher Isw results in a higher reverse recovery current at 150 o C, which indicates that the reverse recovery current consists of both Qoss and Qrr.

Results and Discussion
To clarify the influence of Qoss in the recovery arm on the turn-on characteristics, Fig. 4 exhibits the turn-on waveforms of each configuration at Rg of (a) 50 Ω and (b) 10 Ω at room temperature. It should be noted that the Qrr value in the configuration (A) is negligible at room temperature as discussed in Fig. 3. The Id value during the variation in the drain-source voltage (Vds) is sum of the load current and charging current of the output capacitance in the recovery arm. Therefore, as shown in Fig. 4(a), a higher Id peak of (B) and (C) than that of (A) is observed owing to the higher Qoss of (B) and (C). The larger number of SBDs corresponds to the higher Id peak of (C) than that of (B). Moreover, as shown in Fig. 4(b), the deviation in Id peak between each configuration is higher at a higher switching speed, which can be derived from the faster charging time of the output capacitance in the recovery arm. Fig. 5 shows the relative comparison of Eon of each configuration at Rg of (a) 50 Ω and (b) 10 Ω at room temperature. A higher Qoss results in a higher Eon, and the influence of Qoss is more emphasized for a lower Rg value. A larger impact of the difference in Qoss between each configuration on Eon at a higher switching speed could correspond to the higher deviation in Id peak, as shown in Fig. 4(b).
The turn-off waveforms of each configuration at Rg of (a) 50 Ω and (b) 10 Ω at room temperature are displayed in Fig. 6. As shown in Fig. 6(a), the lower Id of (B) and (C) than that of (A) during the variation in Vds is observed owing to the higher Qoss, or higher discharging current of the output capacitance, of (B) and (C) in the recovery arm. In addition, the deviation in Id between each configuration is higher at a higher switching speed, as shown in Fig. 6(b). Fig. 7 shows the relative comparison of Eoff of each configuration at Rg of (a) 50 Ω and (b) 10 Ω at room temperature. A higher Qoss results in a lower Eoff, and the influence of Qoss is more emphasized for a lower Rg value due to the higher deviation in Id (See Fig. 6(b)). However, unlike the case in the turn-on period, the Eoff values of (B) and (C) are nearly equivalent despite the deviation in Id derived from the difference in Qoss. As can be seen in Figs. 6(a) and 6(b), the rate of change in Vds (dVds/dt) of each configuration also shows some deviation, and a higher Qoss corresponds to a lower dVds/dt. A decrease in Id could have an impact on reducing Eoff, while a decrease in dVds/dt could result in a higher Eoff. Therefore, the influence of the increased Qoss on Eoff is determined by the balance of the decreased Id and dVds/dt.   In order to clarify the influence of Qrr on Eon, the turn-on characteristics of the configuration (A) at room temperature and 150 o C were investigated. Fig. 8 shows the turn-on waveforms of (A) at each temperature at Rg of (a) 50 Ω and (b) 10 Ω. As shown in Fig. 8(a), a higher Id peak at 150 o C than that at room temperature is observed. This phenomenon is owing to the higher Qrr at 150 o C because Qrr is negligible at room temperature, as discussed in Fig. 3, and Qoss is independent of the measurement temperature. The deviation in Id peak is higher in a higher switching speed, as shown in Fig. 8(b), which could be attributed to the faster reverse recovery time. Therefore, as shown in the comparative analysis of Eon at each temperature in Figs. 9(a) and 9(b), the Eon value at 150 o C is higher than that at room temperature at Rg of 10 Ω, while it is lower at Rg of 50 Ω.  The total recovery charge at 150 o C is higher at a higher switching speed. Because Qoss is independent of the switching speed, the observed dependence on Rg could be attributed to the variation in Qrr. This result indicates a more significant impact of Qrr on increasing Eon at a higher switching speed than that of Qoss.

Summary
In this study, we have experimentally revealed a larger impact of Qoss and Qrr on the switching loss of SiC MOSFETs at a higher switching speed. The results indicate the importance of paying attention to the recovery characteristics to ensure a desirable switching loss of SiC MOSFETs.